It is known that some motor vehicles, such as passenger cars or trucks, may be provided with a SCR system for reducing the nitrogen oxides (NOx) emitted by the engine of the motor vehicle. The SCR system generally includes an SCR catalyst which is disposed in an exhaust pipe of the engine to convert nitrogen oxides into diatonic nitrogen and water with the aid of a reducing agent, typically ammonia (NH3). The ammonia may be obtained through the evaporation of an ammonia/water solution, usually referred as diesel exhaust fluid (DEF), which is supplied into the exhaust pipe by a DEF injector located upstream of the SCR catalyst. The DEF may be provided to the DEF injector by a DEF pump which is in fluid communication with a DEF tank installed on board of the motor vehicle.
In order to attain an efficient conversion of the NOx, the concentration of ammonia in the DEF must be greater than a predetermined threshold value. For this reason, some of the strictest anti-pollution regulations require that the SCR systems are provided with a sensor for measuring the ammonia concentration of the DEF contained in the DEF tank, and with an electronic control unit (ECU) configured to activate a maintenance inducement strategy, if the ammonia concentration measured by the sensor is lower than the aforementioned threshold value.
The maintenance inducement strategy generally includes a number of measures that induce the driver of the motor vehicle to restore the efficiency of the SCR system by refilling the DEF tank. However, due to its functioning principle, the ammonia concentration sensor is able to perform a reliable measurement of the ammonia concentration only when it is completely immersed into the DEF. For this reason, when the dynamics of the motor vehicle (e.g. accelerations, deceleration, steering maneuvers, etc.) generates sloshing movements of the DEF that cause the ammonia concentration sensor to occasionally emerge from the DEF, the ECU may temporarily activate the maintenance inducement strategy, even if the SCR system is still perfectly able to convert the nitrogen oxides.
The frequency and the duration of these unnecessary activations of the maintenance inducement strategy usually increase as the DEF level into the DEF tank decreases and, when their impact on the drivability of the motor vehicle becomes unacceptable, the DEF tank must be refilled. However, it has been found that these events call for the refill of the DEF tank long before that the DEF tank is empty (in some cases when 6 liters of DEF are still contained in the DEF tank), with the result that the DEF tank results over dimensioned with respect of the real operating range of the SCR system.